Summary: Sphingomyelin (SM) is one of the major lipid components in plasma and in the cell membranes. Plasma SM level is an independent risk factor for coronary artery disease, and is a marker for the clearance of atherogenic postprandial remnant-like particles. Thus, SM levels have a clinically important impact on lipoprotein metabolism. Moreover, the interaction of SM, cholesterol, and glycosphingolipids drives the formation of plasma membrane lipid rafts. It is well-known that many receptors and transporters are located in these SM-enriched regions, and mediate important cell functions such as cholesterol efflux, lipoprotein secretion and degradation, and inflammatory responses. All these functions may well have an impact on the development of atherosclerosis. Furthermore, we have found that in mouse models, reduction of plasma and tissue SM, with concomitant reduction of atherosclerosis, can be achieved by pharmacological inhibition of SM biosynthesis. This was done by inhibiting serine palmitoyltransferase (SPT), which is the key enzyme for SM biosynthesis. However, the mechanisms involved in this reaction must still be elucidated. All the discoveries made so far emphasize the need for a better understanding of SM metabolism if we are to explore the relationship between SPT deficiency and SM metabolism, as well as SPT deficiency and atherosclerosis. SPT is composed of two subunits, Sptlc1 and Sptlc2 (with Sptlc3 a probable alternative to Sptlc2). Expression levels of Sptlc2 parallel SPT activity, but those of Sptlc1 do not. Overall hypothesis: Sptlc2 deficiency-mediated SM reduction in the circulation and the plasma membranes creates an anti-atherogenic status. Major focus: Manipulating SPT activity using liver- and macrophage-specific Sptlc2-deficient approaches to manipulate SM levels in the plasma and in the cell plasma membranes. This is important because the liver and the macrophages make significant contributions to the development of atherosclerosis. There are two Specific Aims: Aim 1. To evaluate the effect of liver Sptlc2 deficiency on plasma SM metabolism and atherosclerosis, and Aim 2. To investigate the effect of macrophage Sptlc2 deficiency on membrane SM metabolism and atherosclerosis. This proposal will allow us to determine the contributions of hepatic and macrophage SPT activity to SM metabolism and atherosclerosis, and to evaluate SPT as a therapeutic target for atherosclerosis treatment.